The cross flow fan consists of a cylindrical rotor closed at the ends with a cascade of axial blades around its curved surface. The rotor is mounted in a housing which defines the suction and discharge arcs. The housing extends the full length of the rotor and the throughflow is in a plane at right angles to the rotor axis. This fan is a unique type of turbo-machine which differs significantly from axial flow, centrifugal and mixed flow machines, in that each blade passage experiences a continuosly varying flow each time it makes a revolution through the flow field. In this thesis, experimental results are presented which describe the internal flow regime of a large cross flow fan of rotor length 1.0 m and outside diameter 0,625 m. Details of the flow field are examined for a range of dimensionless flow coefficient Ф between 0, 4 and 0.8, which is taken to be the usual operating range for reasons of efficiency and stability of flow. The measurements are transformed into streamlines for the purpose of illustration and the corresponding variations of a dimensionless total pressure coefficient are also given. Flow visualization techniques have been applied to a dynamically similar model to a reduced scale of 1:6.25 operating in water and photographs representing the flow have been examined both quantitatively and qualitatively. A Rankine-type vortex centered near the inner periphery is confirmed but the core was found to remain virtually stationary over the flow range investigated. The total pressure distribution indicates the spatial growth and weakened circulation of the forced vortex core as the flow rate reduces. A depression of total pressure has been identified in a region diametrically opposite the vortex core which intensifies as the flow rate increases. These data are broadly interpreted and are used to improve a previous theoretical model, in which the location of the vortex is used to define the operating point on the fan characteristic.